As known, the knitting lines typically comprise a plurality of yarn feeders each provided with a stationary drum on which a motorized flywheel winds a plurality of yarn loops forming a weft stock. Upon request from a downstream machine, typically a circular/rectilinear knitting machine of a conventional type, the loops are unwound from the drum, then pass through a weft-braking device which controls the tension of the yarn, and finally are fed to the machine.
The yarn feeders of the above type are well-known to the person skilled in the art and have the main scope of maintaining the amount of yarn stored on the drum substantially constant irrespective of the yarn-drawing speed of the machine, while minimizing the tension of the unwinding yarn. To this purpose, the yarn feeder is provided with various sensors, one of which is a loop count sensor, such as an optical sensor, a piezoelectric sensor, and the like, which generates at least one pulse per each unwound loop. This sensor cooperates with the other sensors to optimize the yarn-winding speed of the flywheel, in such a way as to stabilize the amount of yarn stored on the drum.
In the conventional systems, another sensor is arranged between the feeder and the knitting machine for detecting any accidental stops of the yarn, which circumstance may occur in case of breaking of the yarn or unhooking of the yarn from the needles of the machine. In these cases, the control unit stops the machine in order to safeguard the finished article from defects and to prevent the weft tube of the article under processing from detaching, which circumstance, as known, requires a laborious, time-consuming operation of re-inserting all the yarns forming the article into the machine.
As known, the above yarn-breaking sensors may be either mechanical or electronic.
The mechanical sensors have the advantage of being less expensive, but they are also less effective in terms of quickness of response; moreover, they are provided with a sensing arm which grazes the yarn in operation, thereby interfering with the yarn-feeding tension and consequently affecting the accuracy of the tension control system.
The electronic sensors have the advantage of being more effective in terms of quickness of response and, in operation, they do not interfere with the tension of the unwinding yarn because the motion of the yarn is detected by a photoelectric sensor. However, the electronic sensors are very expensive and they require the installation and wiring of an additional supplying/communication circuit, with consequent rise both in costs and in the complexity of the detecting system.
EP-A-200945262 of Applicant describes a method for detecting the stop of the yarn which, in lieu of dedicated breaking sensors, employs the signal generated by the loop count sensor already coupled to the feeder. With the above described method, the interval between the pulses generated by the loop count sensor is compared with a threshold interval which is continuously updated as a function of the changes of the yarn-drawing speed of the downstream machine. When the interval between two pulses exceeds the threshold interval, the system interprets the event as anomalous and stops the machine.
The method described in the above-cited prior document is suitable for those knitting lines in which the yarn is drawn continuously, i.e., the operation of the feeders is never interrupted while forming the pattern. When, on the contrary, the feeders have a discontinuous operation, i.e., they are subjected to stops and restarts, which are typically controlled by respective selectors driven by a cam associated to the rotor of the machine, the above-described method is not suitable because it is not capable of distinguishing any accidental stops from the controlled stops. Typically, knitting lines employing large-in-diameter, so-called “striper” machines, or small-in-diameter, so-called “seamless” machines, or machines for socks, have a discontinuous operation.